The invention relates to a selectable tuner preamplifier with a field-effect transistor tetrode which is comprised of a compound semiconductor. The high-frequency input signal is fed to one gate electrode and the control signal for controlling the high-frequency input signal is fed to the other gate electrode. Band selection is effected by switching off the drain voltage supply.
Such a tuner preamplifier is employed, for example, in a television tuner. FIG. 1 shows the circuit diagram (block diagram) of a television tuner. As is apparent from FIG. 1, the antenna signal is fed to a matching network 1 in a television tuner. The output signal of matching network 1 is fed to the two preamplifiers, i.e., UHF stage 2 and VHF stage 3. Band filter 4 is connected downstream from UHF stage 2 and band filter 5 downstream from VHF stage 3. In addition to the HF signal, UHF stage 2 and VHF stage 3 are fed a further signal, the so-called AGC signal, from AGC stage 6. The AGC signal serves to control (attenuate) the high-frequency input signal. "AGC" is the abbreviation for "automatic gain control". The AGC stage is, for example, an IF integrated circuit. IF is the abbreviation for intermediate frequency. Band selector 7 serves to feed the operating voltage to either UHF stage 2 or VHF stage 3.
A preamplifier such as the UHF stage or VHF stage shown in FIG. 2 comprises, for example, a field-effect transistor tetrode 8 with drain D, source S and gate electrodes G1 and G2. In today's technology, field-effect transistor tetrodes having a semiconductor substrate comprised of a compound semiconductor such a gallium-arsenide are employed to some extent in selectable tuner preamplifiers. Use of field-effect transistor tetrodes comprised of a compound semiconductor in a preamplifier has, for example, the following advantages: higher power gain, less noise, more selectivity, improved large-signal behavior.
In the preamplifier shown in FIG. 2, resistors 9 and 10 constitute a voltage divider for adjustment of the operating point at gate electrode G1 of field-effect transistor tetrode 8. The drain electrode of field-effect transistor tetrode 8 is directly connected to the operating voltage, while resistors 11 and 12 serve to adjust the bias voltage at the source electrode.
If the preamplifier stage shown in FIG. 2 is used, for example, in a television tuner, a band is switched off and a band is selected, respectively. In this case, the drain voltage supply at the field-effect transistor tetrode is switched off.